1. Field of the Invention
The present invention relates to a magnetoresistance effect element using a spin valve film.
2. Description of the Related Art
In general, information recorded on a magnetic recording medium has been read out by a method of detecting a voltage induced in a coil by electromagnetic induction caused by moving a reproducing magnetic head having the coil relatively to the recording medium. On the other hand, it has been known also to use a magnetoresistance effect element (hereinafter referred to as an MR element) in case of reading out information (refer to IEEE MAG-7,150 (1971) and the like). A magnetic head using an MR element (hereinafter referred to as an MR head) utilizes a phenomenon that electric resistance of a ferromagnetic material of some kind varies according to intensity of an external magnetic field.
In recent years, since making a magnetic recording medium smaller in size and greater in capacity has been promoted and a relative velocity between a reproducing magnetic head and a magnetic recording medium when reading out information has been smaller, people's expectation on an MR head which can take out a great output also in a small relative velocity has been increased.
Hereupon, an Ni--Fe alloy, what is called a Permalloy-system alloy has been used in a portion varying in resistance by sensing a magnetic field outside an MR head (hereinafter referred to as an MR element). However, even if a Permalloy-system alloy is an alloy having a good soft-magnetic property, it is at most 3% in magnetoresistance change ratio and therefore it is insufficient in magnetoresistance change ratio as an MR element to be used for a magnetic recording medium which has been made small in size and large in capacity. Accordingly, a magnetoresistance effect material of higher sensitivity has been desired as an MR element material.
In order to meet such a demand, it has been confirmed that a multilayered film where a ferromagnetic metallic film and a non-magnetic metallic film are alternately stacked under a certain condition and where the ferromagnetic metallic films near each other are coupled by antiferromagnetic coupling, what is called an artificial lattice film shows a huge magnetoresistance effect. It has been reported that an artificial lattice film shows a great magnetoresistance change ratio of more than 100% at the maximum (see "Phys. Rev. Lett.", Vol. 61, 2474 (1988), "Phys. Rev. Lett.", Vol. 64, 2304 (1990), and the like). However, since an artificial lattice film has a high saturation magnetic field, it is unsuitable for an MR element.
On the other hand, an example has been reported that a multilayered film of a sandwich structure of a ferromagnetic film/a non-magnetic film/a ferromagnetic film in which the ferromagnetic films are not coupled by antiferromagnetic coupling has also attained a great magnetoresistance effect. Namely, one of the two ferromagnetic films having a non-magnetic film interposed between them is fixedly magnetized by applying an exchange bias to it and the other ferromagnetic film is rotated in magnetization by an external magnetic field (a signal magnetic field or the like). In this way, a great magnetoresistance effect can be obtained by changing a relative angle made between the magnetized directions of the two ferromagnetic films disposed so as to have a non-magnetic film interposed between them. A multilayered film of such a type is called a spin valve film (see "Phys. Rev. B.", Vol. 45, 806 (1992), "J. Appl. Phys.", Vol. 69, 4774 (1991), and the like). Although a spin valve film has a smaller magnetoresistance change ratio in comparison with an artificial lattice film, it is suitable for an MR element thanks to that it can saturate magnetization in a low magnetic field. A great expectation in practical use is placed on an MR head using such a spin valve film.
By the way, in an MR element using the above-mentioned spin valve film, it is said to be important to improve a soft magnetic property of the ferromagnetic film by improving its crystal orientation. For example, in a spin valve film using a Co-based ferromagnetic material such as Co or a Co-based alloy as a ferromagnetic layer, to form a Co-based ferromagnetic layer directly on an amorphous material lowers its crystal orientation and deteriorates its soft magnetic property. Thereupon, it has been studied to improve the crystal orientation by forming a metallic film having an fcc crystal structure as a buffer layer and forming a Co-based ferromagnetic layer on this metallic buffer layer.
However, in case of using a soft magnetic material such as an NiFe alloy or the like as the above-mentioned metallic buffer layer, thermal diffusion easily happens between the soft magnetic material and the Co-based ferromagnetic material, and the magnetoresistance effect is deteriorated. And although it has been studied also to form a ferromagnetic layer to rotate magnetization by an external magnetic field on a magnetic undercoat layer composed of various soft magnetic materials, in such a case also, the magnetoresistance effect is deteriorated due to thermal diffusion which happens between the ferromagnetic layer and the magnetic undercoat layer.
Since a heat treatment is indispensable in a process of manufacturing an MR element using a spin valve film, the above-mentioned deterioration of the magnetoresistance effect caused by thermal diffusion is a serious problem. In such a way, an MR element having a conventional spin valve film has a problem that it is poor in thermal stability (heat resistance), and it has been a great problem to improve the thermal stability by suppressing the thermal diffusion.